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I think one of their methods of finding out your theme (apart from your sites content) is to see who you are linking out to. I wouldnt have thought they would take into account too much about who is linking to you as that is out of your control.
When you do a back link check, you should see sites at the top that relate closely with you. I reckon that if they are totally off topic, Google is a bit confused as to what your site's theme actually is.
I never really link to sites that are off topic now - ok a PR 7 might persuade me but 99/100 I wouldnt bother.
As a run a website that offers information on a business I think 'Would this person be interested in this guy's website if they do not wish to go any further with what I am offering.'
Steve